In the currency coins which were conventionally used in former times, the value of the metal (gold, silver) substantially corresponded to the denomination of the coins so that their actual value changed with the price of the metal.
As industrialization and the intensification of commerce and transport resulted in an increasing demand for coins, that demand was increasingly met since the middle of the nineteenth century by divisional coins having a metal value which usually amounted to only a fractional part of the denomination or impressed value. But even divisional coins made of gold or silver are no longer common at the present time because the metal value has risen so greatly that it has reached or exceeded the impressed value of the coins or because the metal value is at least too high for an economical making of coins.
For this reason, various efforts have been made to replace the orthodox coinage metals gold and silver by other materials. In such efforts it has often been attempted to retain the original color of the coins so that nickel has replaced silver and certain copper alloys have replaced gold.
However, even materials consisting of nickel or of alloys consisting mainly of nickel and used in large amounts as substitutes for silver are now regarded as being too expensive for the making of coins and it is desired to replace them with less expensive materials.
As the substitute materials obviously should have the same performance properties as nickel, aluminum cannot be used because its specific gravity is too low and normal ferrous materials cannot be used owing to their inadequate resistance to corrosion.
But the selection of a suitable substitute material is difficult mainly because the coins made of the new material must be used contemporaneously with coins having the same denomination and made of nickel or nickel alloys at least for some time because the change to the use of the new coins can hardly be effected on a given date from a technical aspect and this would not be reasonable economically. For this reason along the new coins must agree with the old coins in all properties which are detected in automatic coin discriminators. That requirement will have to be met even if a change was to be effected on a specific date because an alteration or substitution of all coin-operated machines to adopt them to the different discriminating properties of the new coins usually cannot be effected in such short order.
Mechanical coin discriminators are used only rarely at present; they test the dimensions of the coin, its density and its rebound behavior upon an impact on an anvil. Such relatively simple tests have been supplemented or replaced in the course of time by the detection of the electrical and magnetic properties of the coins and the coin discriminator can influence the movement of the coins so that counterfeit coins can effectively be distinguished.
Permanent magnets may be provided for retarding electrically conducting but non-magnetizable coins by the braking effect of eddy currents. In that case the ratio of specific density and specific electric conductance of the material of the coin will be critical and a very narrow response range for that ratio can be adjusted.
The movement of magnetizable coins can directly be influenced by magnets. In this case a retardation, acceleration or lateral deflection will affect the coins in most cases as they are flying freely with the result that only coins consisting of the "proper" coinage material will enter the well for receiving acceptable coins.
The discrimination of coins has been improved further in recent times by the provision of so-called electronic coin discriminators. Such a discriminator can consist of an a.c. measuring bridge which in one arm contains a pair of sensor coils, and an electronic device for processing and evaluating the results of measurement. By means of trimming resistors and calibrating coins, the a.c. bridge is so adjusted that its diagonal voltage will be zero when a "proper" coin 5 is disposed between the sensor coils, i.e. a coin which in its dimensions and its material properties agrees with the calibrating coin. That balance indication is used as an indicating signal and is utilized for a decision regarding acceptability. The solenoid coils may be tuned to different frequencies, preferably in the range from 0.1 to 100 kHz. The reliability of the discrimination may greatly be increased by a multiple discrimination at different frequencies. At least two discriminations are usually required for a detection of the influence of the dimensions of the coin in addition to the properties of the material.
For the substitution of nickel by a different material this means that in a discrimination at the conventional frequencies the substitute material must give substantially the same results as nickel.